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152.5.2 Streamflow and Runoff
AR4 concluded that runoff and river discharge generally increased at high latitudes, with some exceptions. No consistent long-term trend in discharge was reported for the world’s major rivers on a global scale. River discharge is unique among water cycle components in that it both spatially and temporally integrates surplus waters upstream within a catchment (Shiklomanov et al., 2010) which makes it well suited for in situ monitoring (Arndt et al., 2010). The most recent comprehensive analyses (Milliman et al., 2008; Dai et al., 2009) do not support earlier work (Labat et al., 2004) that reported an increasing trend in global river discharge associated with global warming during the 20th century. It must be noted that many if not most large rivers, especially those for which a long-term streamflow record exists, have been impacted by human influences such as dam construction or land use, so results must be interpreted with caution. Dai et al. (2009) assembled a data set of 925 most downstream stations on the largest rivers monitoring 80% of the global ocean draining land areas and capturing 73% of the continental runoff. They found that discharges in about one-third of the 200 largest rivers (including the Congo, Mississippi, Yenisey, Paraná, Ganges, Colombia, Uruguay and Niger) show statistically significant trends during 1948–2004, with the rivers having downward trends (45) outnumbering those with upward trends (19). Decreases in streamflow were found over many low and mid-latitude river basins such as the Yellow River in northern China since 1960s (Piao et al., 2010) where precipitation has decreased. Increases in streamflow during the latter half of the 20th century also have been reported over regions with increased precipitation, such as parts of the USA (Groisman et al., 2004), and in the Yangtze River in southern China (Piao et al., 2010). In the Amazon basin an increase of discharge extremes is observed over recent decades (Espinoza Villar et al., 2009). For France, Giuntoli et al. (2013) found that the sign of the temporal trends in natural streamflows varies with period studied. In that case study, significant correlations between median to low flows and the Atlantic Multidecadal Oscillation (AMO; Section 2.7.8) result in long quasi-periodic oscillations. At high latitudes, increasing winter base flow and mean annual stream flow resulting from possible permafrost thawing were reported in northwest Canada (St. Jacques and Sauchyn, 2009). Rising minimum daily flows also have been observed in northern Eurasian rivers (Smith et al., 2007). For ocean basins other than the Arctic, and for the global ocean as a whole, the data for continental discharge show small or downward trends, which are statistically significant for the Pacific (–9.4 km3 yr–1). Precipitation is a major driver for the discharge trends and for the large interannual-to-decadal variations (Dai et al., 2009). However, for the Arctic drainage areas, Adam and Lettenmaier (2008) found that upward trends in streamflow are not accompanied by increasing precipitation, especially over Siberia, based on available observations. Zhang et al. (2012a) argued that precipitation measurements are sparse and exhibit large cold-season biases in the Arctic drainage areas and hence there would be large uncertianties using these data to investigate their influence on streamflow. Recently, Stahl et al. (2010) and Stahl and Tallaksen (2012) investigated streamflow trends based on a data set of near-natural streamflow records from more than 400 small catchments in 15 countries across Europe for 1962–2004. A regional coherent pattern of annual streamflow trends was revealed with negative trends in southern and eastern regions, and generally positive trends elsewhere. Subtle regional differences in the subannual changes in various streamflow metrics also can be captured in regional studies such as by Monk et al. (2011) for Canadian rivers. In summary, the most recent comprehensive analyses lead to the conclusion that confidence is low for an increasing trend in global river discharge during the 20th century.